Silica sol is a colloid solution formed of a dispersion medium and silica particles having a particle size of about 3 to about 100 nm dispersed in the medium, and has high transparency. By virtue of high hardness and heat resistance, such a particulate silica is employed as a resin modifier which is incorporated into resin or a similar material so as to enhance hardness and heat resistance of the resin.
Upon incorporation of silica sol into resin, a silica sol containing an organic solvent or a resin monomer as a dispersion medium is usually used, in consideration of compatibility and reactivity of the sol with the target resin or curing agent. In some cases, surface-hydrophobicized silica powder is employed.
Generally, resins for use in wiring substrates for semiconductor packages and in semiconductor sealing materials contain a silica filler, in order to reduce the linear expansion coefficient of the resins. Conventionally, a micron-size silica filler has been employed for such uses. In recent years, as the pattern pitch of semiconductor elements and the thickness of wiring substrates are reduced, resins for such uses must have lower linear expansion coefficient. Thus, extensive studies have been conducted for increasing the silica filler/resin charge ratio.
When a micro-size filler is incorporated into a resin at high charging ratio, in some cases, the resultant resin composition has reduced flowability to have impaired handling property. When surface-treated, nano-size silica particles are incorporated into such a resin, the flowability of the silica filler-containing resin composition is enhanced (Patent Document 1).
However, since nano-size silica particles have large surface area, the degree of moisture absorption thereof is higher than that of micron-size silica particles, which is problematic. When a highly moisture-absorbing silica filler is incorporated into a resin, the cured resin becomes highly moisture-absorbing, resulting in impairment in insulation performance and mechanical strength. That is, device reliability is impaired. Therefore, low moisture-absorbing property is required for nano-size silica particles.
Meanwhile, in automated semiconductor production steps, the position of each device must be detected automatically. Thus, a semiconductor sealing material or adhesive, in particular a sealing resin, is required to have high transparency. Such a sealing resin is also required to have low linear expansion coefficient, and there is demand for nano-size silica particles which can be incorporated into a sealing epoxy resin at high charging ratio.
In addition, the silica filler for the aforementioned such electronic material uses is desired to have a very small radioactive element (e.g., uranium or thorium) content, since α-ray emitted by radioactive elements (e.g., uranium and thorium) may cause mulfunctions of semiconductor devices.
Generally speaking, silica sol is produced from sodium silicate as a raw material or from silicon alkoxide as a raw material. When produced from sodium silicate as a raw material, the produced silica sol contains large amounts of radioactive elements (e.g., uranium and thorium). Among the radioactive elements, thorium is difficult to remove even through ion exchange of sodium silicate. Thus, difficulty is encountered in application, to the aforementioned electronic material uses, of a silica sol produced from readily available sodium silicate as a raw material.
In contrast, silicon alkoxide is a raw material which has high silicon alkoxide purity. Thus, silicon alkoxide can readily provide a silica sol having a low level of radioactive elements in colloidal silica, which is advantageous. However, no satisfactorily low-moisture-absorbing silica particles have ever been produced, and application of moisture-absorbing silica particles to the aforementioned electronic material uses problematically causes impairment in device performance.
Several nano-size silica fillers are commercially available, and examples include fumed silica powder and molten method silica powder. These commercial products advantageously have low moisture-absorbing property, but have considerably poor resin dispersion property. Thus, problems such as stickiness of resin varnish occur, thereby failing to attain high charging ratio.